Wave filter



W. P. MASON Feb. 21, 1939.

WAVE FILTER Filed Feb. 19

NORMAL ROCHELLE SALT DESICCATED ROCHELLE SALT I I II .2 RATIO OF WIDTHTO LENGTH FIG? /Nl EN7'0/? By W. P. MASON FREQUENCY ATTORNEY PatentedFeb. 21, 1939 UNITED STATES PATENT OFFICE WAVE FILTER ApplicationFebruary 19, 1936, Serial No. 64,696

25 Claims.

This invention relates to wave filters and more particularly to the useof Rochelle salt crystals as piezoelectric impedance elements in suchfilters.

An object of the invention is to extend the frequency range in whichpiezoelectric crystals may be used as reactance elements in wavefilters.

Other objects are to increase the Width. of transmission band obtainableand to reduce the cost of such filters.

A further object is to decrease the variation in attenuation withtemperature changes in filters of this type.

Heretofore, quartz crystals have been used as piezoelectric impedanceelements in wave filters having transmission bands located down to about50 kilocycles. However, due to the comparatively small electromechanicalcoupling exhibited by quartz, wide bands centered at frequencies below50 kilocycles are obtainable only at the sacrifice of attenuation in theregions of suppression.

In accordance with the present invention, Rochelle salt crystals areadapted for use in wave filters having transmission bands centered inthe frequency range from 20 to 60 kilocycles. In the preferredembodiment, the impedance elements are rectangular plates so cut from aRochelle salt crystal that the two principal faces of the plate areperpendicular to one of the principal axes of the crystal, with a lengthdimension which makes angles of 45 degrees with the other two axes, andhaving a width to length ratio lying between 0.35 and 0.55. In order tomake the attenuation of the filter more stable with changes intemperature, the electrodes are spaced apart from the crystal plate, or,if integral electrodes are used, the plating is applied only to thecentral portions of the major faces of the crystal, the portions nearthe ends being left unplated. Also, in order to preserve the crystalsthey are placed in a container in which the relative humidity iscontrolled by exposing therein a quantity of desiccated and a quantityof normal Rochelle salt.

Due to the large electromechanical coupling obtainable in Rochelle saltcrystals, filters having comparatively wide bands and highdiscrimination outside the bands may be provided. When the constructionoutlined above is employed, the filters are sufiiciently stable withtemperature changes, and the crystals remain in operating condition foran indefinite time.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawing, of which:

Fig. 1 is a perspective view of a homogeneous Rochelle salt crystalshowing the principal axes;

Fig. 1a shows the orientation of the piezoelectric element with respectto the axes of the crystal;

Fig, 2 is a perspective View of a piezoelectric Rochelle salt crystalelement with its associated electrodes suitable for use in the wavefilter of the invention;

Fig. 3 is an end View of a Rochelle salt crystal element with associatedelectrodes spaced from the crystal;

Fig. 4 is a set of curves showing the frequency constant and frequencyspectrum for a Rochelle salt crystal element such as shown in Fig. 2 or3;

Fig. 5 shows two Rochelle salt crystal elements IIlOlll'llJGd within asealed container with means for maintaining a constant relative humiditytherein;

Fig. 6 is a schematic circuit of a lattice type wave filter usingRochelle salt crystals as component impedance elements; and

Fig. '7 shows diagrammatically a typical attenuation characteristicobtainable with the Wave filter of Fig. 6.

Fig. 1 shows in perspective a typical homogeneous Rochelle salt crystalsuitable for supplying the piezoelectric impedance elements used in thewave filter of the invention. The crystal has three mutuallyperpendicular faces A, B, C, and three principal axes a, b, c which areperpendicular, respectively, to the A, B and C faces. The impedanceelement consists of a parallelepiped or rectangular plate so cut fromthe R0- chelle salt crystal that a principal face of the plate isperpendicular to one of the principal axes of the crystal. Fig. 1ashows, by way of example, such a plate I having a face 2 which isperpendicular to the crystal axis b. In the preferred form the plate isso oriented that its major axis, that is, a length dimension, makes anangle of approximately 45 degrees with the other two principal axes ofthe crystal. In Fig. 1a the segment 3 shown in dotted outline has thisorientation, the major axis 4 making an angle of approximately 45degrees with the a and c applied. If integral electrodes are employed.it is found desirable to apply them only to the central portion,covering approximately 70 per cent of each principal face, while about15 per cent at each end is left unplated, in order to stabilize theanti-resonant frequency as a function of temperature. Fig. 2 is aperspective view of such a partially plated crystal element ll having; aprincipal face it? of length Z and width w to which has been applied twosymmetrically placed electrodes l3 and i i. The electrodes may be madeof gold or other suitable metal, preferably applied by a sputteringprocess. It is to be understood that two other similar electrodes areplaced on the other principal face, that is, the one opposite to faceI2. A divided plating is used on each face so that a single crystalelement may be made to provide portions of two different impedancebranches in a wave filter, as explained more fully hereinafter inconnection with Fig. 6.

When vibrating in the longitudinal mode the element I I has a transversenodal line at the center along which the crystal is preferablysupported. As shown in Fig. 2 the supporting means may comprise a pairof clamps l5, E6 on one-side and a second pair of similar clamps, notshown, oppositely disposed on the other side between which the crystalplate is held. The clamps may be made of conducting material, or ofother material faced with conducting material, and the electricalconnections may be made to the clamps or directly to the electrodes.

Another method found to be very effective in stabilizing all of theproperties of the crystal element, and especially the anti-resonantfrequenoy, as a function of temperature is to separate the electrodes ashort distance from the crystal, leaving a small air gap therebetween. Aseparation of the order of 0.001 inch has been found to give goodresults. One way of providing the desired spacing is to make thesurfaces of the crystal slightly rough and to allow the elec trodes torest directly thereon. Or, alternatively, spacers of insulating materialmay be inserted between the electrodes and the crystal. The lattermethod is illustrated in Fig. 3 which shows an end view of a crystalelement H with electrodes ll, I8 associated with one principal face andelectrodes i9, 20 associated with the opposite face. The area of theelectrodes may cor- 7 respond substantially with the area of the twoprincipal faces of the crystal. The electrodes may, for example, beplates of glass or other material which will retain its form, on theinner side of which is deposited a coating of conducting material, asshown at 2!. The conducting material is platinum, aluminum, or someother suitable metal, applied preferably by a sputtering process. Eachplate is spaced from the crystal a distance d by means of: a spacerwhich may, for example, be a deposit of non-conducting cement, shown at22, located at the nodal line. 'The crystal and its associatedelectrodes may be supported by means of suitable clamps, not shown,which contact the outer faces of the electrodes.

In order to provide a crystal element which is sufficiently free fromextraneous resonances and one which is comparatively stable withtemperature changes it hasbeen found desirable to holdv the ratio-1 ofthe width w tothe length l of each principal face, as indicated in Fig.2, within certain limits. One. reason for this maybe deduced from a;study of the curvespresentedin Fig. 4, which relate to a Rochelle saltcrystal element having a principal face out perpendicular to the a axis,with a length dimension making an angle of 45 degrees with the b and caxes. The resonance frequencies for the various modes of vibration, inkilocycles per second per centimeter of length Z, are shown plottedagainst the ratio r. The solid line curve 23 represents the principalmode of vibration, the extensional mode in the direction of the lengthZ, and .curve 24 represents the extensional mode in the direction of thewidth to. The dotted line curves 25, 25 and 21 represent extraneousshear vibrations, of which the only one of consequence is 25, due to itsproximity to curve 23. It will be noted that curves 23 and 25 have theirwidest separation near the point where r is 0.4 and therefore this isthe optimum ratio in this respect. However, for values of r,

ranging approximately from 0.35 to 0.55 it has been found that there isa sufficient separation between the two curves to insure satisfactoryoperation of the crystal element in wave filters. The range of r lyingbetween 0.20 and 0.25 can also be, utilized, but the elements have animpedance which is too high for some purposes. The ranges of-r lyingjustbelow 0.2, just below 0.3 and between 0.6 and 0.7 where other curvescross curve 23-, should particularly be avoided. The region in which ris greater than 0.7 should also be avoided because of the proximity hereof the shear curves.

Rochelle salt crystal elements having a principal face cut perpendicularto the b axis, as shown in Fig. la, also have the type of frequencyspectrum shown in Fig. 4, and therefore when this type of element isused the ratio r should be held within the same limits as thosementioned above.

It has also been found that approximately the same range of r, namelyfrom 0.35 to 0.55, insures the greatest stability of frequency withchanges in temperature, and therefore this is a second reason foremploying crystal elements which have principal faces having theserelative proportions.

In order to protect the crystal it is placed in a reasonably air-tightcontainer such as- 28 shown in perspective, partly cut away, in Fig. 5.There is shown within the container two crystal elements 29 and 30having integral electrodes of the type shown in Fig. 2'. The crystalsare mounted in a holder which is supported from one side of thecontainer by means of two metal rods such as 3|. The holder comprisesthree members 32, 33 and 34 made of insulating material in which areembedded four pairs of clamps such as 35 and 36' each held in place by ametal pin such as 31. The crystal plates 29 and 30 are mounted betweenthe four pairs of clamps as shown, and electrical connections to theelectrodes are made by means of connectors such as 38 one end of whichis soldered to the pin 31. Each connector passes through a hole in thecontainer and is insulated therefrom by means of an insulating sealcomprising a flanged metallic cylinder 39 partially filled with a glassbead 40 through which passes the connector. The flange of the cylinder39is soldered to the side of the container so as to provide a jointwhich will not admit air or moisture.

In order to preserve the crystal it is desirable that the relativehumidity of the surrounding atmosphere be controlled within certainlimits even though changes in temperature occur. This is accomplished byexposing within the container a small quantity of desiccated and a smallquantity of normal Rochelle salt. Preferably these salts are granulated,and may be separately placed in two cloth cartridges such as 4| and 42supported by means of string between the two uprights 43 and 44, asshown in Fig. 5. It has been found that one cubic inch of each of thesalts is sufficient to control the humidity of two cubic feet of airwithin the sealed container, when the temperature range encountered isnot too great. If the temperature changes are not too rapid or too greata comparatively constant humidity will be maintained. When thetemperature is lowered and the relative humidity inside the containertends to increase, the excess moisture is absorbed by the desiccatedsalt, thus maintaining the equilibrium. On the other hand, if thetemperature rises the relative humidity will tend to decrease, but thenormal salt gives up moisture, again maintaining the equilib rium.

Fig. 6 is a schematic circuit diagram showing how two Rochelle saltcrystal elements 45 and 46 of the type illustrated in Fig. 2 or 3 eachhaving two pairs of electrodes may be utilized to provide a portion ofeach of the four impedance branches of a lattice-type wave filter havinga pair of input terminals 41, 48 and a pair of output terminals 49, 50.The pair of electrodes 5|, 52 associated with element 45 are connecteddirectly between terminals 41 and 49 to form part of a series impedancebranch, and the other pair of electrodes 53, 54 are connected betweenterminals and 48 to form part of the other series branch. The two pairsof electrodes associated with element 46 are connected, respectively,between the input terminals and the output terminals to form thediagonal impedance branches. The filter is completed by the addition ofthe two series capacitances C1, the two shunt capacitances C2 and thefour series inductances L1, as shown. The component reactance elementsmay be proportioned so that the filter will have a band-passtransmission characteristic, with one or more attenuation peaks on eachside of the band, of the type shown in Fig. '7. It is sometimes foundthat the attenuation in the regions of suppression may be increased byproviding an inductive coupling between each inductance at one end ofthe filter and the corresponding inductance. at the other end.

What is claimed is:

l. A wave filter comprising a plurality of impedance branches arrangedbetween a pair of input terminals and a pair of output terminals, one ofsaid impedance branches comprising as a re actance element a Rochellesalt crystal, in the form of a plate having a width to length ratiofalling between 0.35 and 0.45, said ratio being so chosen as to providethe widest separation between the resonance frequency of said crystalwhen vibrating in the extensional mode in the direction of its lengthand the resonance frequency of the nearest extraneous shear vibration.

2. A wave filter comprising a plurality of impedance branches arrangedbetween a pair of input terminals and a pair of output terminals and soproportioned with respect to each other and to two preassignedfrequencies that said filter will transmit the band lying between saidfrequencies, one of said branches comprising as an impedance element aplate cut from a Rochelle salt crystal, said plate having a faceperpendicular to the b axis of said crystal, the major axis of saidplate making an angle of approximately 45 degrees with each of the otherprincipal axes of said crystal, and said plate having a length so chosenthat said plate exhibits a longitudinal resonance at a frequency withinthe operating range of said filter.

3. A wave filter comprising as an impedance element a crystalline plateof Rochelle salt having integral electrodes which cover the centralportions of its two principal faces leaving the end portions of saidfaces uncovered, a sealed container enclosing said plate, and meanswithin said container for maintaining therein a constant relativehumidity over a temperature range.

4. A wave filter comprising as an impedance element a plate cut from aRochelle salt crystal, said plate having a width to length ratio sochosen that the utilized frequency of vibration predominates over allextraneous frequencies of vibration, a sealed container surrounding saidelement, and a quantity of normal Rochelle salt and a quantity ofdesiccated Rochelle salt exposed within said container whereby therelative humidity within said contaner is maintained within specifiedlimits while the temperature varies over the operating range.

5. In a wave filter, an impedance element comprising a crystalline plateof Rochelle salt and associated electrodes, a principal face of saidplate having a width to length ratio of approximately 0.4, whereby thenearest extraneous resonance is located in the frequency spectrum amaximum distance from the vibration frequency of said plate in itsprincipal mode of vibration.

6. In a wave filter, an impedance element comprising a crystalline plateof Rochelle salt, an electrode supported adjacent a face of said plate,said electrode being separated from said plate by a short distance toprovide an air gap therebetween in order to stabilize the vibrationfrequency of said element as a function of temperature, a sealedcontainer enclosing said plate and means within said container forstabilizing the relative humidity with changes in temperature.

7. A piezoelectric impedance element comprising a plate cut from aRochelle salt crystal and two electrodes, one of said electrodes beingassociated with each major face of said plate, said electrodes beingseparated from said plate by spacers which contact said plate only oversmall areas at the nodal line, and said electrodes being spacedapproximately 0.001 inch from said plate in order to minimize changes inthe vibration frequency of said element with changes in temperature.

8. A piezoelectric impedance element comprising a plate cut from aRochelle salt crystal and electrodes associated therewith, said elementhaving a principal face of length Z and width w, the ratio of w to Zlying between 0.35 and 0A5 and being so chosen as to provide the widestseparation between the resonance frequency of said element whenvibrating in the extensional mode in the direction of its length and thefrequency of the nearest extraneous shear vibration.

9. A piezoelectric impedance element consisting of a plate cut from aRochelle salt crystal, each of the two principal faces of said elementhaving a symmetrically placed integral electrode covering approximately'70 per cent of the surface at the center, leaving approximately 15 percent at each end unplated in order to minimize changes in the vibrationfrequency of said element with changes in temperature.

10. A piezoelectric element comprising a crystalline plate of Rochellesalt, an electrode supported adjacent a face of said plate with aseparation of approximately 0.001 inch between said plate and saidelectrode, a sealed container enclosing said plate, and means withinsaid container for maintaining therein a constant rela- I tive humidityover a temperature range.

11. A piezoelectric impedance element comprising a plate cut from aRochelle salt crystal and 0.25, and said electrodes coveringapproximately 70 per cent of the surface at the center of the twoprincipal faces of said plate, leaving approximately 15 per cent at eachend uncovered in order to minimize changes in the vibration frequency ofsaid element with temperature variations.

14. In a piezoelectric device, a crystalline plate of Rochelle salt.having a width to length ratio lying between 0.35 and 0.45, said ratiobeing so chosen 'as to provide the widest separation between theresonance frequency of said plate when vibrating in the extensional modein the direction of its length and the frequency of the nearest shearvibration.

15. In a piezoelectric device, a crystalline plate of Rochelle salthaving a width to length ratio of approximately 0.4, and electrodesassociated with the two principal faces of said plate, said electrodescovering approximately per cent of the surface at the center, leavingapproximately 15' per cent at each end uncovered whereby the vibrationfrequency of said plate is stabilized with respect to temperaturechanges.

16. A resonator comprising an element cut from a piezoelectric crystaland an electrode associated with a face of said element, said electrodeconsisting of a glass plate the inner surface of which is coated withconducting material, and said electrode being separated from saidcrystal element by a deposit of non-conducting cement.

17. A piezoelectric element comprising a plate cut from a Rochelle saltcrystal, said plate having a face perpendicular to the b axis, of saidcrystal, the major axis of said plate making an angle of approximately45, degrees with each of the other principal axes of said crystal, andsaid face having a ratio of width to length so chosen as to provide theWidest separation between the resonance frequency of said element whenv1- brating in the extensional mode in the direction of its length andthe resonance frequency of the nearest shear vibration.

18. A piezoelectric device comprising a crystalline plate of Rochellesalt having integral electrodes whichcoverthe centralportions of'itstwo.principal faces leaving the endportions. of said faces uncovered, asealed container enclosing said: plate, and means within said containerfor stabilizing the relative humidity withv changes in temperature.

19. An impedance element comprising a piezoelectric crystal of Rochellesalt and a pair ofconductive coatings on two opposite faces of saidcrystal, said coatings. covering approximately 70 per cent of thesurface of said faces at the center, leaving approximately 15 per centat each end uncoated in order to stabilize the vibration frequency ofsaid element with temperature variations.

20. An impedance element comprising a piezoelectric crystal of Rochellesalt and a pair of-conductive coatings on two opposite faces of saidcrystal, said coatings covering only the central, portions of saidfaces, leaving at least 15 per cent of the surface at each end uncoatedin order to stabilize thevibration frequency of said element withtemperature variations.

21'. A piezoelectric element in accordance with claim 17 which includesmeans for supporting said plate in the region of the nodal line.

22. A wave filter in accordance with claim 2 in which said impedancebranches are arranged to form a lattice network.

23. A wave filter in accordance with claim 2 in which said face. has aratio of width to length so chosen as to provide the widest separationbetween the resonance frequency of said element when vibrating in theextensional mode in thedirection of its length and the resonancefrequency of the nearest shear vibration.

24. A wave filter comprising two pairs of equal impedance branchesconnected between two in,- put terminals and two output terminals toform a symmetrical lattice network, and a plate cut from a Rochelle saltcrystal, said plate having two pairs of oppositely disposed electrodesby means of which it is connected to each of one pair of equal branches,said plate having a principal face perpendicular to the b axis of the.crystal, the major axis of said plate making an angle of approximately45 degrees with the a and c axes, and said plate having a length sochosen that said plate exhibits a longitudinal resonance at a frequencywithin the operating range of said filter.

25. A wave filter comprising two pairs of equal impedance branchesarranged to form a symmetrical lattice, a crystalline plate of Rochellesalt symmetrically coupled electro-mechanically to both branches of oneequal pair and a second similar plate similarly coupled to the otherpair of equal branches, said plate having different resonancefrequencies and each of said plates being cut from a Rochelle saltcrystal perpendicular to the b axis and with its longest dimension at anangle of approximately 45 degrees to the a and c axes.

WARREN P. MASON.

